As the density of integrated circuits has increased, there is increasing need to reduce the size of associated capacitors in, for example, Dynamic Random Access Memories (DRAMs). However, there is also a need to increase the per unit area capacitance of such capacitors, which may be difficult given that the overall size of the capacitors may be reduced for use in highly integrated circuits. For example, it has been known to use SiO2 dielectric layers in Metal-Insulator-Semiconductor (MIS) capacitors using a three-dimensional structure to compensate for the reduced thickness of the dielectric layer. In other words, a three-dimensional structure has been used to increase the effective surface area of the electrodes despite the reduction in size of the capacitor. Such MIS conventional capacitors, however, may not be suitable for use in all highly integrated circuits.
To address some of the shortcomings of MIS capacitors, it is known to use metal-insulator-metal (MIM) capacitors where the dielectric layer is formed using a metal oxide that has a high oxygen affinity. For example, it is known to use metal oxides such as Ta2O5, Y2O3, La2O5, Pr2O3, Nb2O5, TiO2, BaO, SrO, HfO2 and BST. In particular, an HfO2 layer can have a relatively high dielectric constant (e.g., about 20-25) and a high band gap. These properties of the HfO2 dielectric layer (unlike other dielectric layers having high-dielectric constant) may provide relatively good reliability and stability in integrated circuit memory devices.
FIG. 1 is a graph showing leakage currents in a conventional MIM type capacitor. In particular, the capacitor leakage currents shown in FIG. 1 correspond to a MIM type capacitor having TiN electrodes and an HfO2 dielectric material therebetween. According to FIG. 1, the curve labeled (A) shows that the leakage current may be relatively low after initial formation of the MIM type capacitor. However, curves (B) and (C) illustrate that the leakage current of the MIM type capacitors after anneal of 500° C. and 550° C., respectively. The leakage currents can be increased due to back-end processing, which can include forming an inter-layer dielectric, a barrier metal layer, and an inter-metal dielectric layer, after the formation of the MIM type capacitor. The formation of a barrier metal may take place at a relatively high temperature of 550° C. to 700° C.
The increase in leakage current described above can occur due to the crystallization of the HfO2 layer and/or a reaction between the HfO2 layer and the TiN electrodes. Furthermore, differences in the thermal expansion of the electrodes and the dielectric layer can also contribute to increased stress in the capacitor. All of these factors can contribute to an increase in the equivalent oxide thickness of the dielectric layer, which in turn can contribute to an increase in leakage current.
The crystallinity of HfO2 layers is known to depend on the post treatment, especially, on temperature and method. FIG. 2 shows the amount of crystallization in the HfO2 layer immediately after formation of a 60 Å-thick HfO2 dielectric layer (Curve A), after thermal processing of a 60 Å HfO2 layer in a vacuum (Curve B) at 650° C., and after plasma processing of a 60 Å HfO2 layer in an H3 plasma at 390° C. (Curve C). According to FIG. 2, the HfO2 layer subject to plasma processing at 390° C. demonstrates about the same amount of crystallization as that exhibited by dielectric layers subject to a high temperature thermal processing. Even the crystallinity of HfO2 layers after both treatments is the same, the leakage current behavior is considerably different because of the lower temperature in plasma treatment.